Liquid heaters and vaporizers – Separators – Interior of boilers
Reexamination Certificate
2000-10-06
2001-10-16
Wilson, Gregory (Department: 3749)
Liquid heaters and vaporizers
Separators
Interior of boilers
C122S488000, C122S379000
Reexamination Certificate
active
06302064
ABSTRACT:
The invention relates to a steam generator, particularly a pressurized-water nuclear reactor steam generator, which comprises a baffle for distributing feedwater in the steam generator.
Pressurized-water nuclear reactors comprise steam generators which are used for heating and vaporizing the feedwater, using the heat conveyed by the water used to cool the nuclear reactor.
The steam generator comprises a shell of cylindrical overall shape arranged, in service, with its axis vertical, and in which the feedwater is heated up and turned into steam. The nuclear reactor cooling water is made to flow through the steam generator inside the tubes of a bundle arranged inside a bundle wrapper which is fixed in a coaxial configuration inside the steam generator shell. Each of the tubes of the bundle generally comprises two straight legs, the ends of which are each crimped into an opening passing through a tube plate fixed transversely inside the steam generator shell, that is to say in an arrangement at right angles to the axis common to the shell and to the bundle wrapper of the steam generator, which constitutes the axis of the steam generator.
With the shell, the bundle wrapper of the steam generator delimits an annular space extending in the vertical axial direction of the steam generator, and the steam generator comprises means for introducing and distributing water into the upper part of the annular space. In addition, the bundle wrapper has a lower edge located a certain distance above the top face of the tube plate, so that the feedwater introduced into the upper part of the annular space flows first of all from the top downwards in the annular space, then enters the bundle wrapper via the opening formed between the lower part of the bundle wrapper and the tube plate to then flow from the bottom upwards inside the bundle wrapper in contact with the exterior surface of the tubes through which the nuclear reactor cooling water is circulated.
The feedwater is thus gradualy heated up and turned into steam as it flows in contact with the tubes of the bundle.
The steam obtained at the exit of the steam generator is sent into the nuclear power station turbine and is then recovered in the form of condensate which is recycled through the secondary circuit of the steam generator through which the feedwater flows. As it flows through the secondary circuit and through the steam generator, the feedwater becomes laden with solid matter such as oxides in the form of particles in suspension in the feedwater. These particles have a tendency to become deposited in those zones of the secondary circuit in which the feedwater flows at low speed. In particular, this suspended matter is liable to become deposited on the top face of the tube plate, which produces a deflection of the streams of feedwater coming from the peripheral annular space, so as to direct the feedwater from the periphery towards the central part of the steam generator and so as to gradually direct the streams of water in the vertical and upward direction, in contact with the tubes of the bundle.
The particles of suspended matter tend to be deposited in certain zones of the tube plate in which the flow speeds are the lowest, particularly in the central part of the tube plate.
The tubes of the bundle each of which comprises two straight legs are held inside the wrapper by transverse tube support plates, that is to say plates which are at right angles to the axis of the wrapper and spread out in the axial direction of the steam generator, so that the straight legs of the tubes, which legs are engaged in the openings passing through the tube support plates are held parallel to the axis of the steam generator and arranged in grid configurations in the transverse planes of the wrapper. The openings passing through the tube support plates are made in such a way as to be able, on the one hand, to hold the tubes of the bundle in transverse directions and, on the other hand, to allow feedwater to pass through the successive tube support plates, in contact with the tubes of the bundle.
To limit the zones at which the feedwater flows at low speed in contact with the tube plate, which are zones at which matter in suspension in the feedwater is potentially deposited, it has been proposed that a baffle for distributing the flow of feedwater be arranged in the bottom part of the bundle wrapper, this baffle having, like the tube support plates, openings for the passage of tubes of the bundle and being made in such a way as to encourage the flow of feedwater over the tube plate, particularly from the periphery towards the central part of the bundle. For that, the baffle for distributing the flow of feedwater may be made in such a way as to have a circular hole in its central part, and arranged at a well-defined height above the tube plate.
The two legs of each of the tubes of the bundle are fixed by crimping and welding their ends into the holes passing through the tube plate arranged in a grid structure similar to the grid of the openings in the tube support plates. The holes passing through the tube plate are distributed on each side of a hole-free central zone, so that an empty space or central tube lane is formed inside the bundle, vertically in line with that zone of the tube plate which has no holes, and occupies a diametral zone of the tube plate.
To prevent a high flow rate of feedwater from flowing down the steam generator tube lane, it has been proposed that blocking-off devices be placed inside the tube lane to force the feedwater towards the zones occupied by the tubes of the bundle.
In the case of devices such as those described, the openings for the passage of tubes passing through the tube support plates and the holes passing through the flow distribution baffle are made by piercing and/or broaching and may have a cross section of threelobed or four-lobed shape, that is to say a cross section which allows them to take and hold a tube of the bundle in a central part, extended radially by three or four lobes which allow the cooling water to pass around the tube.
The use of a flow distribution baffle like the one described above therefore encourages a flow of feedwater in which a substantial proportion of the feedwater which is directed towards the central part of the tube bundle passes through the flow distribution baffle via its central opening, at an appreciably higher speed than in the peripheral zone of the baffle. This use of a flow distribution baffle with a central hole makes it possible to limit the deposits on the tube plate but on the other hand may encourage the matter in suspension to be carried along in the feedwater towards those zones of the tube bundle which are located above the flow distribution baffle and towards the tube support plates.
To limit as far as possible the extent to which suspended matter is carried along in the feedwater towards the upper parts of the tube bundle, a solution that makes it possible to obtain a more uniform distribution of speeds throughout the lower part of the steam generator has been sought.
The object of the invention is therefore to provide a steam generator, particularly a pressurized-water nuclear reactor steam generator, used for heating and vaporizing the feedwater flowing in thermal contact with a hot fluid circulated to the inside of tubes of a bundle, each having two straight legs, comprising a shell of cylindrical overall shape, a tube plate fixed in an arrangement that is perpendicular to the axis of the shell and pierced with openings to take the ends of the straight legs of the tubes of the bundle, a bundle wrapper arranged coaxially inside the shell and inside which is placed the bundle of tubes, the straight legs of which are parallel to the axis of the steam generator common to the shell and to the bundle wrapper and a lower edge of which is arranged above the tube plate, a number of transverse plates which are mutually parallel and parallel to the tube plate, fixed in arrangements which are spread out in the direction of the axis of the steam generator, inside the bundl
Billoue Jean-Paul
Cornu Bernard
Destre Daniel
Connolly Bove Lodge & Hutz
Framatome
Wilson Gregory
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